Semiconductor-on-insulator integrated circuit with interconnect below the insulator

ABSTRACT

An integrated circuit fabricated on a semiconductor-on-insulator transferred layer is described. The integrated circuit includes an interconnect layer fabricated on the back side of the insulator. This interconnect layer connects active devices to each other through holes etched in the insulator. This structure provides extra layout flexibility and lower capacitance, thus enabling higher speed and lower cost integrated circuits.

BACKGROUND OF THE INVENTION

In an integrated circuit, metal lines typically connect individualcircuit elements together. The nature of this metal interconnectcritically affects the performance and cost of the integrated circuitproduct. For example, most integrated circuit processes offer multiplelevels of metal interconnect, in order to allow the maximum flexibilityin circuit layout. This layout flexibility allows a designer to minimizethe size of an integrated circuit, for example, reducing the cost of theproduct.

An example of a typical circuit layout with two levels of metalinterconnect is shown in FIG. 1. This layout includes twometal-oxide-semiconductor field effect transistors (MOSFET) 240.Isolation edges 235 and gate fingers 240 g define the transistors. Oneof the transistors 240 has two gate fingers 240 g and one has three gatefingers. The gate shapes 240 g and isolation edges 235 determine thesource and drain regions of the transistors (240 s and 240 d,respectively). The drains 240 d of both transistors 240 are electricallyconnected together, through contacts 245, first metal lines 250,inter-metal vias 270, and second metal line 280. The source regions 240s of the two transistors are connected individually through contacts 245and first metal lines 250. The layout of FIG. 1 could be, for example, acomplementary metal-oxide-semiconductor (CMOS) inverter, if one of thetransistors 240 is an n-channel MOSFET and the other is a p-channelMOSFET.

In most multi-level metallization schemes, to connect upper-level metallines to lower level metal lines or transistor electrodes (sources,drains, and gates), all of the intermediate metal layers and contacts orvias must be used. This can be seen in FIG. 1, where the second levelmetal line 280 is connected to transistor drains 240 d through the firstmetal layer 250 and metal/diffusion contact 245. Thus, to connect sourcefingers 240 s together, for example, a metal line 250 runs outside thetransistor area to avoid shorting transistor sources 240 s and drains240 d together. This increases the height, and thus the area, of thelayout. Moreover, the width of this layout is determined by the minimumpitch of the contacted first metal lines, or the minimum pitch of thecontacted source/drain regions—whichever is greater. If the minimumpitch of the contacted first metal lines is the greater of the twopitches, then this layout may be reduced in width by an alternativemetallization scheme. Note that the minimum pitch of the contacted metallines may be determined by lithographic or other process considerations,or it may be determined by electromigration concerns or otherconsiderations/concerns.

The resistance (per unit length) and capacitance (per unit area) of themetal interconnect layers often has a direct impact on the performanceof an integrated circuit. The interconnect structures and materials usedaffect, in turn, the resistance and capacitance of the interconnectlines. For example, the capacitance between a given interconnect lineand the semiconductor substrate, or the capacitance between twointerconnect lines, decreases as the vertical distance between themincreases. If multiple interconnect layers are available, criticalcapacitances can be reduced by placing their nodes on metal layers thathave more vertical separation between them.

To illustrate the origin of these parasitic capacitances, across-section of the layout of FIG. 1 is shown in FIG. 2. Parasiticcapacitances, for example, between the gates 240 g and the drain regions240 d (gate-drain capacitance) and the source 240 s and drain 240 dregions (off-state capacitance), are determined partly by theinterconnect structure. For example, the overlap area between the firstmetal lines 250 and the source regions 240 s forms a component of theoff-state parasitic capacitance 290. The capacitance between the contactmetal 245 and the transistor gates 240 g contributes to the totalgate-drain parasitic capacitance. Thus, the metallization scheme andlayout has an effect on circuit parasitic capacitances, and thereforecircuit performance.

These parasitic effects are important for high frequency and high-speedcircuits. Such circuits are often implemented onsemiconductor-on-insulator (SOI) technology, which was firstcommercialized in the late 1990s. The defining characteristic of SOItechnology is that the semiconductor region in which circuitry is formedis isolated from bulk substrate by an electrically insulating layer.This insulating layer is typically silicon dioxide. The advantageousaspects of SOI technology stem directly from the ability of theinsulator layer to electronically isolate the active layer from bulksubstrate.

SOI technology represents an improvement over traditional bulk substratetechnology because the introduction of the insulating layer isolates theactive devices in an SOI structure, which improves their electricalcharacteristics. For example, parasitic capacitances within thesemiconductor region—depletion and diffusion capacitances, forexample—are often reduced in SOI devices, especially those with thinenough semiconductor layers such that the transistors are“fully-depleted.” Short-channel effects—the variation of the thresholdvoltage of a transistor with its channel length—are also reduced in SOIdevices. For these reasons, among others, SOI is often the technology ofchoice for high-speed, low-power circuits. In addition, the SOI'sinsulating layer can act to shield the active devices from harmfulradiation. This is particularly important for integrated circuits thatare used in space given the prevalence of harmful ionizing radiationoutside the earth's atmosphere.

Like bulk-substrate technologies, SOI technologies may also make use ofmultiple layers of metal interconnect. If these metal layers can beformed in such a way as to take advantage of the unique structure ofSOI, cost and performance benefits can result. These performancebenefits may be especially desirable for the types of circuits typicallyfabricated on SOI technologies—high-speed or high switching frequency,low-loss circuits.

SUMMARY OF THE INVENTION

An integrated circuit assembly comprising a transferredsilicon-on-insulator structure is described. The structure comprises aninsulating layer having a first surface and a second surface, and a holeextending from the first surface to the second surface. A semiconductorlayer has a first surface and a second surface, and its first surfacecontacts the first surface of the insulating layer. The semiconductorlayer contains active or reactive circuit elements. A metal interconnectlayer is coupled to the second surface of the insulating layer, themetal interconnect layer being disposed within the hole in theinsulating layer. The hole in the insulating layer extends to at leastthe first surface of the semiconductor layer, and the metal interconnectlayer electrically couples a plurality of the active or reactive circuitelements together.

A method of forming a transferred semiconductor-on-insulator integratedcircuit is described. The semiconductor-on-insulator includes aninsulating layer having a first surface and a second surface, asemiconductor layer having a first surface and a second surface, and asubstrate layer. The first surface of the semiconductor layer contactsthe first surface of the insulating layer. The substrate layer contactsthe second surface of the insulating layer. Active or reactive circuitelements are formed in the semiconductor layer. A handle layer iscoupled to the second surface of the semiconductor layer. The substratelayer is removed, and a hole extending from the second surface of theinsulator layer to its first surface is formed in the insulating layer.The hole exposes the first surface of the semiconductor layer. A metalinterconnect layer is formed on the second surface of the insulatorlayer and inside the hole in the insulator layer. The metal interconnectlayer electrically couples a plurality of the active or reactive circuitelements to each other.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a typical integrated circuit layout with two layersof metal.

FIG. 2 illustrates a cross-section of the circuit layout shown in FIG.1.

FIG. 3 shows a process flow chart of a method of fabricating anintegrated circuit with metal interconnects on the back side of an SOIinsulator that is in accordance with a specific embodiment of thepresent invention.

FIGS. 4A-G show cross-sectional views of stages of forming interconnecton the back side of an SOI circuit, according to some embodiments.

FIG. 5 shows a cross-sectional view of another embodiment, whereinmultiple types of back contacts are formed.

FIG. 6 shows a layout of an SOI integrated circuit using back sideinterconnect that is in accordance with a specific embodiment of thepresent invention.

FIGS. 7A-B show an alternative layout and cross section of an SOIintegrated circuit using back side interconnect that is in accordancewith a specific embodiment of the present invention.

FIG. 8 shows a cross-sectional view of transistors formed using anembodiment of the current invention, illustrating differences inparasitic capacitances.

FIGS. 9A-C show cross-sectional views of stages of forming back sidediffusions and interconnect, according to some embodiments.

FIGS. 10A-C show cross-sectional views of stages of forming back sidecontacts and interconnect, according to some embodiments.

DETAILED DESCRIPTION OF THE EMBODIMENTS

High-speed, high performance integrated circuits are often fabricatedusing silicon-on-insulator (SOI) technologies. SOI technologies alsolend themselves to layer-transfer techniques, where the active layer ofthe SOI is transferred, using bonding and etch-back methods, to adifferent substrate. This method may have advantages of lowercapacitance and higher speed, easier thermal management, and easieraccess to the body regions of SOI transistors. An example of such aprocess is described in U.S. Pat. No. 8,357,935 entitled,“Silicon-on-insulator with back side connection”, owned by the assigneeof the present application and filed on Apr. 28, 2012, which isincorporated herein by reference in its entirety. Layer transfertechniques may also be used to form circuits on fully insulating ortransparent substrates, which may be advantageous in opto-electronicapplications, for example.

Transferring the active layer of an integrated circuit also can exposethe surface of the insulator layer (opposite the active devices) tofurther processing, if desired. In particular, a full interconnect layercan be formed on this insulator layer surface after the substrate isremoved. An interconnect layer so placed may contact the transistorsource and drain regions through holes etched in the SOI's insulatinglayer. This arrangement offers added layout flexibility. Also, this backside interconnect layer provides lower parasitic capacitance whencompared to traditional, front-side interconnect layers. Lastly, such aninterconnect layer aids in dissipating heat generated by the SOIintegrated circuit. The problems with heat dissipation on SOI circuits,and some proposed solutions, are described in U.S. Patent PublicationNo. 2011/0012199 entitled, “Semiconductor-on-insulator with back-sideheat dissipation,” owned by the assignee of the present application andfiled on Jul. 14, 2010, which is incorporated herein by reference in itsentirety.

In one embodiment, a full back side SOI interconnect layer connects aplurality of active or reactive devices together, providing a lower costalternative for providing high performance SOI integrated circuits.Various embodiments include: reducing contact resistance to the backside of the active layer by forming heavily-doped regions through theback side contacts, or etching through the active layer to contactheavily doped or silicided areas on the front surface of the activelayer. If the transferred structure can withstand higher temperatures,more process flexibility is possible.

FIG. 3 illustrates some embodiments of methods of the presentdisclosure, where a metal interconnect layer is formed on the back sideof an SOI integrated circuit. In flowchart 100 of FIG. 3, asemiconductor-on-insulator structure is provided in step 110. Thisstructure includes an insulator layer disposed between a semiconductorlayer and a substrate layer. The insulator layer may be, for example,silicon dioxide, which may be 15-1000 nm thick. In step 120, active orreactive circuit elements are formed in the semiconductor layer. Suchelements may include, for example, n-channel and p-channel MOS (NMOS andPMOS) transistors. The semiconductor layer may be removed in certainareas—for example, in the isolation areas located between activedevices—and replaced with dielectric. These elements may be formedusing, for example, a standard CMOS process, or a bi-polar-CMOS (BiCMOS)process, or a process that forms high-power devices or optoelectronicdevices in addition to MOS transistors. This process may include forminga metal-semiconductor compound on regions of the semiconductor layer;for example, on the source and drain regions of the MOSFETs. Such acompound would reduce the sheet resistance of these regions. If thesemiconductor layer comprises silicon, the metal-semiconductor compoundmay comprise, for example, titanium silicide, cobalt silicide, or nickelsilicide. In step 130, a metal interconnect layer is optionally formedabove the semiconductor layer. This layer may electrically connect tothe circuit elements formed in the semiconductor layer. This layer maybe formed using a subtractive, or Damascene, process, or it may beformed using an additive process. This layer may be comprised, forexample, of a refractory metal, for example, tungsten.

In step 140 of FIG. 3, a handle layer is coupled to the semiconductorlayer of the SOI structure. The handle layer could be any rigid orsemi-rigid support, for example, a silicon wafer. Any suitable couplingor bonding method that results in a permanent bond may be used; forexample direct or fusion bonding, permanent adhesive bonding, metallicinterdiffusion or eutectic bonding. If a front-side interconnect layeris used (step 130), it may be advantageous for bonding purposes to use aDamascene process to form this layer, since a planar surface willresult. A bonding layer, for example, silicon dioxide, may be depositedon the SOI structure, or the handle layer, or both. This layer may bedeposited using, for example, chemical vapor deposition (CVD). In step150, the substrate layer of the SOI is removed. This could be done byusing, for example, mechanical and chemical means independently or incombination.

Still referring to FIG. 3, in step 160, a hole is etched in the exposedsurface of the insulator layer. This hole extends through the insulatorlayer, from one surface to the opposite surface. This may beaccomplished, for example, by forming the hole pattern in a photoresistusing photolithography, and etching the insulator layer in an RIE orplasma etch chamber, or using hydrofluoric acid (HF). After the hole isformed, the semiconductor layer surface is exposed inside the hole.During the etching step 160, another hole that extends to a transistorgate layer, or to the (optional) interconnect layer formed in step 130,may be formed. Such a hole could be formed, for example, through thedielectric isolation areas between the active or reactive devices. Allof these different types of holes—those extending to the semiconductorlayer, a gate layer, or an interconnect layer—are etched through thesame material (for example, silicon dioxide). Thus, they can all beetched in the same step 160. In step 170, the hole extending to thesemiconductor layer may optionally be extended into the semiconductor.The etching may stop at the metal-semiconductor compound layer, ifpresent. This etching process may be done in a dry-etch chamber, using achemistry that will etch the semiconductor faster than themetal-semiconductor compound. For example, for a silicon layer withnickel silicide formed on the front surface, an SF₆+O₂ etch will etchthe silicon but not etch the nickel silicide.

In step 180 of FIG. 3, a metal interconnect layer is formed on the backside of the insulator and inside the hole formed in step 160. This metalinterconnect layer could comprise, for example, tungsten plugs,aluminum, copper, or a combination of these. It could be formed bystandard patterning techniques; for example, depositing the metal,patterning with a photoresist, and etching the metal. The resultingpattern will connect two or more of the devices formed in thesemiconductor layer to each other. If there are holes formed in step 160that extend to a gate layer or a front-side metal interconnect layer (ifstep 130 is performed), then an electrical connection can be formedbetween the back side metal formed in step 180 and a gate or front sidemetal layer.

FIGS. 4A-G illustrate an exemplary back side SOI structure fabricatedaccording to the method of FIG. 3. In FIG. 4A, an SOI structure 200,having a substrate layer 210, an insulating layer 220, and asemiconductor layer 230, is provided. The insulating layer 220 has afirst surface 223 (in contact with the semiconductor layer 230) and asecond surface 226 in contact with the substrate 210. The semiconductorlayer 230 has a first surface 233 (in contact with the first surface 223of the insulating layer 220) and a second surface 236. The insulatinglayer may be comprised of, for example, silicon dioxide, and it may be,for example, between 10 and 1000 nm thick, for example, between 15 and70 nm thick, or between 150 and 350 nm thick, or between 500 and 750 nmthick. The semiconductor layer may be comprised of, for example,silicon, or a III-V semiconductor such as GaAs, or a strainedsemiconductor alloy such as SiGe.

In FIG. 4B, circuit elements, for example, transistors 240, are formedin the semiconductor layer. These transistors 240 comprise source 240 sand drain 240 d regions, and gate layers 240 g, and are separated byisolation regions 232. These elements may be formed as described forstep 120 in FIG. 3 above; that is, using, for example, a standard CMOSprocess, or a bi-polar-CMOS (BiCMOS) process. Other circuit elements maybe formed in addition to, or instead of, CMOS transistors 240, forexample, high-power devices, optoelectronic devices, or other passive orreactive elements.

The source 240 s and drain 240 d regions in FIG. 4B comprise areas ofthe semiconductor layer that are heavily doped, in comparison, forexample, to the transistor channel regions underneath the gates 240 g.The high doping level is important, for example, to allow goodelectrical contact to these regions. If the semiconductor layer is thinenough—for example, less than 100 nm, or less than 70 nm, or less than50 nm—the source and drain heavily-doped regions may extend through thefull thickness of the semiconductor layer, as shown in FIG. 4B. Notealso that the source 240 s and drain 240 d regions of transistors 240may be raised above the surface 236 of the semiconductor layer 230. Sucha structure is often used for fully-depleted SOI processes, which arebuilt on very thin semiconductor layers, for example, less than 20 nm,or less than 10 nm, or between 5 and 7 nm.

FIG. 4C shows a front metal interconnect layer 250 optionally coupled tothe second surface 236 of the semiconductor layer 230, and above thecircuit elements (transistors 240) fabricated therein. If present, thisinterconnect layer 250 may connect, for example, to the source regions240 s of the transistors 240. A subtractive, or Damascene, process maybe used to form optional interconnect layer 250. Interconnect layer 250may comprise a high-temperature capable interconnect, for example, arefractory metal, for example, tungsten. Alternatively, interconnectlayer 250 may comprise a conventional low-resistivity material, forexample, copper or aluminum, or it may comprise a combination ofhigh-temperature-capable interconnect and low-resistivity material, forexample tungsten plugs with aluminum or copper metal. An inter-leveldielectric layer 258 separates the optional front metal interconnectlayer 250 from the devices 240 formed in the semiconductor layer 230. InFIG. 4D, a handle layer 260 is coupled to the surface 236 of thesemiconductor layer 230, with inter-level dielectric layer 258 and frontinterconnect layer 250, if present, intervening. As shown in FIG. 4D,handle layer 260 is bonded above interconnect layer 250, if present. Asdescribed in step 140 of FIG. 3, the handle layer could be any rigid orsemi-rigid support, for example, a silicon wafer, and any suitablecoupling or bonding method that results in a permanent bond may be used;for example direct or fusion bonding, or permanent adhesive bonding. Abonding layer (not shown), for example, deposited silicon dioxide, maybe formed on the either, or both, of the bonded surfaces.

In FIG. 4E, the substrate 210 of the SOI structure has been removed, byusing, for example, mechanical and chemical means independently or incombination. This step exposes the surface 226 of the insulator layer220. In FIG. 4F, holes 270 have been etched through the insulator layer220, exposing regions of the surface 233 of the semiconductor layer 230.These holes can be formed by any standard patterning technique asdescribed above for step 160 in FIG. 3, for example, photolithographicpatterning of a resist layer followed by dry etching of the insulatorlayer 220. These holes may expose, for example, the drain regions 240 dof transistors 240. Holes could also expose, for example, source or bodyregions of CMOS transistors, or collector regions of vertical bipolartransistors, or collector, base, or emitter regions of lateral bipolartransistors.

Turning to FIG. 4G, a back side metal interconnect layer 280 is formedon the surface 226 of insulator layer 220 and inside the holes 270. Asdescribed in step 170 of FIG. 3, this metal interconnect layer couldcomprise, for example, aluminum, copper, tungsten, or a combination ofthese. It could be formed by standard patterning techniques; forexample, an additive process, or a subtractive process. The back sidemetal layer 280 may connect two or more transistors 240 to each other.For example, as shown in FIG. 4G, back side metal layer 280 may connectsome of the drains 240 d of transistors 240 together.

FIG. 5 shows an alternative structure that may be formed using theprocess according to the method of FIG. 3. In FIG. 5, other holes 272and 274 are formed, in addition to holes 270. Both holes 272 and 274 areformed over isolation regions between transistors 240. Hole 272 extendsto a gate region 248 that extends over the isolation region, and hole274 extends to the optional front metal interconnect layer 250. Holes272 and 274 may be formed simultaneously with holes 270; that is, in thesame masking step. Alternatively, separate masking steps may be used forthe different types of contact holes, for example, if it is desired thatthe overetch of the semiconductor layer inside holes 270 be minimized.In this structure, the back side interconnect is electrically connectedto the optional front side interconnect, or the transistor gate layer,or both. This can allow greater layout flexibility and thus costsavings.

FIG. 6 shows an example layout that utilizes the area-saving features ofthis invention. As in FIG. 2, gate fingers 240 g and isolation edges 235define transistors 240. Transistors 240 further comprise drain regions240 d and source regions 240 s. A front side metal interconnect layer250 connects the source regions 240 s together through contacts 245.Note that there is no connection between interconnect layer 250 and thedrain regions 240 d; therefore, the metal layer 250 connecting thesource regions 240 s can be drawn overlapping the drain regions 240 dwithout going outside of the transistor area. Drain regions 240 d areconnected through back side holes (or vias) 270 to back sideinterconnect layer 280. In this way, the area of this integrated circuitcan be reduced compared to the prior art shown in FIG. 2.

FIG. 7A shows an example layout for the reduction of parasiticcapacitance. Again, gate fingers 240 g and isolation edges 235 definethe source 240 s and drain 240 d regions of transistors 240. Front sidemetal 250 is again connected to the source regions 240 s throughcontacts 245. Back side metal 280 is connected to drain regions 240 dthrough back side vias 270. So as to reduce gate-to-source (off-state)capacitance, the source metal 250 is not drawn overlapping the drainregions 240 d, and the back side drain metal 280 is not drawnoverlapping the source regions 240 s. Thus, the interconnectcontribution to the parasitic off-state capacitance is limited to thearea of overlap of the back side metal 280 and front side metal 250layers. However, this capacitance is much reduced compared to the caseof FIG. 2, since the two layers are more separated vertically. FIG. 7Bshows a cross sectional view of the layout illustrated in FIG. 7A. Thisfigure clearly shows how the front side source metal 250 and the backside drain metal 280 are separated from each other as far as they canbe.

Using this embodiment, where the back side metal 280 connects to drainregions 240 d, the gate-to-drain capacitance is also reduced. FIG. 8shows a cross section of two transistors 240 x and 240 y fabricatedusing an embodiment of the current invention. Transistor 240 x has itsdrain 240 d contacted from the front, and transistor 240 y has its drain240 d contacted from the back. The gate-drain capacitance for transistor240 x includes as a component the capacitance 295 between the frontcontact 245 and the gate 240 g. However, for the back-contactedtransistor 240 y, there is no such capacitance adding to the totalgate-drain capacitance. Thus, the total gate-drain capacitance isreduced for transistors with back-contacted drains.

Note also that, in the transistor layouts (FIG. 6 and FIG. 7), thetransistors 240 may be more compacted in the direction perpendicular tothe gate fingers 240 g, compared to layouts using standard processes(e.g., FIG. 2). This is the case if the minimum front-side contactedmetal pitch is greater than the minimum contacted transistorsource/drain pitch, which would make the layout of FIG. 2 metal-pitchlimited. As shown in FIGS. 6 and 7, the number of front side metalpitches needed to contact the sources and drains of the transistors 240is reduced by about a factor of 2.

FIGS. 9A-C illustrate an alternative embodiment of an SOI back sidemetal interconnect. FIG. 9A shows an SOI integrated circuit, withtransistors 240 and front-side interconnect layer 250, bonded to handlelayer 260. Heavily doped source regions 240 s and drain regions 240 d donot extend through to the back surface 233 of semiconductor layer 230.Holes 270 have been etched in insulator layer 220 to expose surface 233of semiconductor layer 230. In FIG. 9B, dopant has been introducedinside holes 270 to form doped regions 241, of the same dopant type asdrain regions 240 d. Regions 241 effectively extend drain regions 240 dto the back surface 233 of semiconductor layer 230, and allow the drainregions 240 d to be contacted through back side holes 270. Doped regions241 may be formed by, for example, implanting dopant ions through holes270, as described in U.S. Patent Publication No. 2012/0088339 A1entitled, “Vertical Semiconductor Device with Thinned Substrate,” ownedby the assignee of the present application and filed on Oct. 11, 2011,which is incorporated herein by reference in its entirety.Alternatively, doped regions 241 may be formed by exposing the structureto a dopant-species-containing ambient (for example, POCl₃) at a hightemperature (for example, greater than 800° C.). If ion implantation isused, it may be followed by an annealing process to activate the dopant,for example, a rapid thermal anneal at a temperature of 800 to 1000° C.for 1 to 60 seconds. Note that, if a high temperature process is usedfor this step, it would be advantageous to use a high temperature metalfor the front side metal interconnect layer 250, if this layer is used.For example, it would be advantageous if a refractory metal, forexample, tungsten, were used for front side metal interconnect 250.

In FIG. 9C, a back side metal interconnect layer 280 is formed on thesurface 226 of insulator layer 220 and inside the holes 270, contactingdoped region 241 connected to drain regions 240 d. This metalinterconnect layer could comprise, for example, tungsten, aluminum,copper, or a combination of these. This interconnect layer may, as shownin FIG. 9C connect the drains 240 d of transistors 240 together, throughback side vias 270 and doped regions 241.

The process described in FIGS. 9A-B provide a means of forming aback-side interconnect contacting drain or source regions, when theseregions do not extend to the back side of the semiconductor layer. Thismay be the case, for example, if the semiconductor layer is greater than200 nm thick, or greater than 100 nm thick, or greater than 70 nm thick.

FIGS. 10A-C illustrate another alternative embodiment of an SOI backside metal interconnect. FIG. 10A shows an SOI integrated circuit, withtransistors 240 and front-side interconnect layer 250, bonded to handlelayer 260. Heavily doped source regions 240 s and drain regions 240 d donot extend through to the back surface 233 of semiconductor layer 230.Moreover, regions of a metal-semiconductor compound 242 have been formedon the surfaces of the drain regions 240 d, source regions 240 s, andgate regions 240 g of transistors 240. These metal-semiconductorcompounds are often used to reduce the sheet resistance and contactresistance of these regions. If the semiconductor layer comprisessilicon, the metal-semiconductor compound may comprise, for example,titanium silicide, cobalt silicide, or nickel silicide. These regionsare often referred to as “self-aligned silicide”, or “SAlicide”,regions.

Still referring to FIG. 10A, holes 270 have been etched in insulatorlayer 220 to expose surface 233 of semiconductor layer 230. In FIG. 10B,the holes 270 have been extended into the semiconductor layer 230 tocontact the back side of the metal-semiconductor compound regions 242.This etching process may be done in a dry-etch chamber, using achemistry that will etch the semiconductor faster than themetal-semiconductor compound. For example, for a silicon layer withnickel silicide formed on the front surface, an SF₆+O₂ etch will etchthe silicon but not etch the nickel silicide. In FIG. 10C, a back sidemetal interconnect layer 280 is formed on the surface 226 of insulatorlayer 220 and inside the holes 270, contacting metal-semiconductorcompound regions 242 electrically connected to source regions 240 s.This metal interconnect layer could comprise, for example, tungsten,aluminum, copper, or a combination of these.

The structure of FIG. 10C offers several advantages. For example, thetransferred semiconductor layer 230 does not need to be exposed to hightemperature steps (e.g., to activate dopants), so that low-resistivity,low-temperature metals (e.g., aluminum or copper) may be used for thefront-side metal interconnect layer, if present. Also, the hole 270filled with interconnect metal 280 contacting the metal-semiconductorcompound region 242 together provide an excellent thermal path forefficient transfer of heat from active devices 240. Finally, the contactresistance between the metal-filled hole 270 and the metal-semiconductorcompound 242 may be advantageously lower than the metal-semiconductorcontact resistance between metal-filled holes 270 and the siliconsurface 233 (FIG. 4G and FIG. 9C).

In FIG. 10C, the back side interconnect layer 280 is shown connectingtransistor sources 240 s rather than transistor drains 240 d. In thisstructure as shown, there may be some conduction between the metalfilling the hole 270 and the body region 240 b of the transistor 240.Thus, in FIG. 10C, the sources and body regions of transistors 240 maybe effectively shorted together. For many circuits, especially digitalcircuits, this is desired. For these types of circuits, then, thisstructure may offer a layout advantage, since a separate body contact isnot necessary.

The process described in FIGS. 10A-C may also be used in combinationwith the process wherein the diffused regions reach the back side of thesemiconductor layer 230 (FIGS. 4A-G). This may be advantageous, forexample, in cases a lower drain contact resistance is desired. Thisstructure is also less susceptible to unintended conduction between themetal filling the holes 270 and the body regions of transistors 240, so,for example, analog or other circuits, where source-body ties are notnecessarily desired, may be implemented.

The back-side interconnect processes described herein are applicable tomany different types of SOI fabrication processes, for example,fully-depleted or partially-depleted SOI processes. Also, multiplelayers of metal interconnect may be used on the front or back side ofthe transferred layer, by, for example, repeating the applicable stepsdescribed herein.

While the specification has been described in detail with respect tospecific embodiments of the invention, it will be appreciated that thoseskilled in the art, upon attaining an understanding of the foregoing,may readily conceive of alterations to, variations of, and equivalentsto these embodiments. These and other modifications and variations tothe present invention may be practiced by those of ordinary skill in theart, without departing from the spirit and scope of the presentinvention. Furthermore, those of ordinary skill in the art willappreciate that the foregoing description is by way of example only, andis not intended to limit the invention. Thus, it is intended that thepresent subject matter covers such modifications and variations.

What is claimed is:
 1. A method of forming an integrated circuitassembly, the method comprising: providing a semiconductor-on-insulatorincluding: an insulating layer having a first surface and a secondsurface; a semiconductor layer having a first surface and a secondsurface, wherein the first surface of the semiconductor layer contactsthe first surface of the insulating layer; and a substrate layercontacting the second surface of the insulating layer; formingtransistors in the semiconductor layer; coupling the second surface ofthe semiconductor layer to a handle layer; removing the substrate layer;forming a hole in the insulator layer, wherein the hole in the insulatorlayer extends from the second surface to the first surface of theinsulator layer and exposes the first surface of the semiconductorlayer; and forming a metal interconnect layer on the second surface ofthe insulator layer, wherein the metal interconnect layer is disposedwithin the hole in the insulating layer, and wherein the metalinterconnect layer electrically couples a plurality of the transistorsto each other.
 2. The method of claim 1, further comprising, before thestep of coupling the second surface of the semiconductor layer to ahandle layer, forming a second metal interconnect layer coupled to thesecond surface of the semiconductor layer.
 3. The method of claim 2,wherein the second metal interconnect layer comprises a refractorymetal.
 4. The method of claim 1, wherein the step of forming transistorsin the semiconductor layer comprises forming a plurality of heavilydoped regions extending from the second surface to the first surface ofthe semiconductor layer.
 5. The method of claim 4, wherein the hole inthe insulator layer exposes at least one of the heavily doped regionsextending from the second surface to the first surface of thesemiconductor layer.
 6. The method of claim 1, further comprising, afterthe step of forming the hole in the insulator layer, introducing anactive dopant into the semiconductor layer through the hole.
 7. Themethod of claim 6, wherein the step of introducing an active dopant intothe semiconductor layer comprises: implanting the dopant through thehole; and annealing the structure.
 8. The method of claim 1, wherein thestep of forming transistors in the semiconductor layer comprises formingregions of a metal-semiconductor compound on the second surface of thesemiconductor layer; and further comprising, after the step of formingthe hole in the insulator layer, extending the hole in the insulatorlayer by removing a portion of the semiconductor layer within the holeso as to expose the region of the metal-semiconductor compound.
 9. Themethod of claim 8, wherein the metal-semiconductor compound comprises ametal silicide.
 10. The method of claim 1, wherein the semiconductorlayer comprises silicon.
 11. The method of claim 1, wherein the step offorming transistors in the semiconductor layer comprises forming afield-effect transistor; and wherein the hole in the insulating layerexposes a source region or a drain region of the field-effecttransistor.
 12. The method of claim 1, wherein the step of formingtransistors in the semiconductor layer comprises forming field-effecttransistors; and wherein the metal interconnect layer electricallycouples only to drain regions of the field-effect transistors.
 13. Themethod of claim 1, wherein the plurality of transistors are electricallycoupled to each other via the metal interconnect and an additionalcircuit element.